Bleaching of lignin-containing cellulose materials such as pulp

ABSTRACT

Digester pulp is brightened and delignified by contacting the pulp with an aqueous medium containing oxygen and trisodium phosphate. The oxygen liberates the lignin substituents and the trisodium phosphate carries the lignin substituents into solution while minimizing degradation of the polymeric structure of the cellulose.

United States Patent Farley [54] BLEACHING OF LIGNIN-CONTAINING CELLULOSE MATERIALS SUCH AS PULP [72] Inventor: Charles Edward Farley, Fairfield, Conn. [73] Assignee: American Cyanamid Company, Stamford,

Conn.

[22] Filed: Dec. 21,1970

21 Appl. No: 100,474

Related U.S. Application Data [63] Continuation-in-part of Ser. No. 24,363, Mar. 31,

1970, abandoned.

[52] U.S. Cl ..l62/65, 8/111, 162/80, 162/85 [51] Int. Cl ..D2lc

[58] Field ofSearch ..l62/65,80, 85; 8/111 [4 1 May 9, 1972 Primary E.\'aminerHoward R. Caine Attvrne vEvans Kahn ABSTRACT Digester pulp is brightened and delignified by contacting the pulp with an aqueous medium containing oxygen and trisodiurn phosphate. The oxygen liberates the lignin substituents and the trisodium phosphate carries the lignin substituents into solution while minimizing degradation of the polymeric structure of the cellulose.

4 Claims, No Drawings BLEACHING OF LIGNIN-CONTAINING CELLULOSE MATERIALS SUCH AS PULP This is a continuation in part of my copending application Ser. No. 24,363 filed on Mar. 31, i970 and now abandoned.

The present invention relates to the brightening and delignification of cellulose digester pulp. More particularly, the invention is an improvement in the brightening and delignifying of such pulp by dissolved molecular oxygen.

Digester pulp is a moist pulp of cellulose fibers which have lignin and perhaps other color bodies chemically attached thereto. It is known that this cellulose (hereinafter termed lignocellulose") is brightened and delignified when subjected to the action of compressed oxygen in the presence of sodium hydroxide and magnesium carbonate; cf. U. S. Pat. Nos. 3,024,158, 3,251,720 and 3,384,533, and French Pat. Nos. 1,810,248 and 1,404,605. In the process, the oxygen liberates the lignin substituents and oxidizes them to some extent and the sodium hydroxide solubilizes them, and the magnesium carbonate acts to inhibit depolymerization of the cellulose.

The process has the disadvantages of failing to separate all the lignin substituents and of causing unduly much depolymerization of the cellulose, so that the cellulose is less useful than desired for the manufacture of paper. Moreover, the treatment requires three materials, and this is inconvenient.

The discovery has now been made that cellulose digester pulp can be brightened and delignifled by a process which consists essentially in contacting the pulp with oxygen while the aqueous medium of the pulp has a dissolved content of a tri-(alkali metal) phosphate. In this step sodium hydroxide, magnesium carbonate and other similar materials heretofore deemed necessary are not used; they are either not beneficial or they are disadvantageous.

In preferred embodiments the process possesses the following advantages.

1. Substantial increases in brightening and in lignin liberation are achieved, and the cellulose undergoes less degradation. The process therefore produces a purer and more valuable cellulose, particularly for the rayon, cellophane and paper manufacturing industries.

2. The process does not require high oxygen pressures so that present equipment can be used.

3. The aqueous medium need not be discarded after use but can be regenerated by addition of a commonly available material (sodium hydroxide) and recycled. As a result, loss of unconsumed reagents and ecological contaminators are minimized.

More in detail, according to the present invention cellulose digester pulp is admixed with an aqueous solution of trisodium (or tripotassium) phosphate, and after the phosphate has become uniformly distributed through the pulp the pulp is subjected to the action of dissolved molecular oxygen at normal or at elevated pressure. The treatment is continued until a desired proportion of lignin has been liberated, after which the pulp may be washed and used for the manufacture of paper with or without further bleaching. Preferably but not necessarily, as much as possible of the aqueous medium is separated from the pulp and is regenerated and reused. Because of its low cost, trisodium phosphate is preferred as the phosphate and the invention will hereinafter be described in terms of this material (which is sometimes designated herein as TSP").

In the process, the primary functions of the aqueous medium are to place the fibers in direct contact with dissolved molecular oxygen and the trisodium phosphate, and to carry off the solubilized and oxidized lignin substituents. In the laboratory we have experienced good results by treating the fibers in the form of loose flocs containing sufiicient water to prevent combustion. We have also experienced good results when the fibers were treated in the form of a highly mobile pumpable slurry (fiberzwater weight ratio 1:33, equivalent to a consistency of 3 percent). Our data indicate that fastest flocking occurs when the fiberzwater weight ratio is about 1:5 to about 1:10. It does not appear, however, that the amount of aqueous medium present is critical in any instance so long as there is sufficient water to wet the fibers and so provide a transfer medium for the oxygen and a receptor medium for the released lignin substituents, and to prevent combustion.

The water used for the preparation of the trisodium phosphate solution need not be fresh water but may be water which contains such agents as are commonly present in papermill water systems including pentachlorophenol, dodecylguanidine, tributyl tin oxide and other biocides; inert salts including sodium chloride, sodium aluminate and sodium sulphate; titanium dioxide, barites and other pigments which are inert in the process; rosin size; cellulose fiber fines; and recycled oxidized lignin in soluble salt form. These materials are tolerated in moderate amounts and do not alter the essential character of the process by which the lignin substituents are liberated, oxidized and solubilized. Recycled papermill white water may therefore be used for the preparation of the trisodium phosphate solution.

The minimum amount of trisodium phosphate in the solution which is mixed with the pulp and which is required for best results is that which is sufficient to solubilize the oxidized lignin which is formed by the process. This lignin in free acid form is insoluble in water, but becomes soluble by treatment with trisodium phosphate which converts it to soluble salt form. Since different pulps yield different proportions of oxidized lignin when treated, the minimum effective amount of trisodium phosphate varies from instance to instance. However, a suitable amount in any instance can be found by making a series of laboratory trials as shown below. To prevent the loss that would result from too little trisodium phosphate I prefer to use an excess. A substantial excess is tolerated well, and in practice I generally use an amount within the range of 5 percent to 25 percent of trisodium phosphate based on the weight ofthe solution, which generally provides this excess.

In the process the oxygen pressure may be as low as 2 lb./in. this being about the minimum which makes the process economically worth while. Faster and more complete liberation is efiected (with comparatively little depolymerization of the cellulose) in the range of 50-150 lb./in. which therefore is preferred. Still higher pressures can be used, but the amount of depolymerization of the cellulose becomes unacceptably high for most uses, with little saving in time and with little gain in the amount of lignin liberated.

An oxygen pressure ofless than 14 lb./in. sq. (i.e., less than atmospheric) occurs when oxygen (in pure form or in admixture with one or more other gases) is applied to the solution while the solution is at elevated temperature. The minimum oxygen pressure occurs when the solution is at or near the boil. The oxygen is conveniently applied by passing oxygen in pure form or as a gaseous mixture over or through the solution to be treated. The oxygen dissolves rapidly. lf preferred, the oxygen can be pumped into the solution while it is in an autoclave under pressure at a temperature well above normal.

The process yields good results at low temperatures (e.g. 60 C.) but faster and more extensive delignification and brightening occur at elevated temperatures (e.g. in the range of 75 1 50 C. up to the point where significant decomposition of the cellulose commences.

The oxygen may be supplied in pure form or as air. Pure oxygen is preferred as this avoids the higher pressures needed when air is used.

The process can be applied commercially by spraying sheets of digester pulp having a water content of 20 30 percent by weight (as is normal) with sufficient of a concentrated solution of trisodium phosphate to provide 1 to 20 percent of trisodium phosphate based on the wet weight of the pulp, allowing the sheets to stand for a few minutes until the trisodium phosphate has become uniformly distributed therethrough, and subjecting the sheets to the action of oxygen at an appropriate temperature (for example, 50 lb./in. at C. for 30 minutes).

The sheets may then be squeezed and washed to recover as much as possible of the aqueous medium, after which they may be further bleached and baled for shipment. Removal of the solubilized oxidized lignin occurs when the pulp sheets are disintegrated in water and the fibers are formed into paper. The solubilized oxidized lignocellulose leaves with the water.

If preferred, the fibers can be treated in the form of a pumpable slurry which eliminates need for the handling of moist webs. In this alternative a slurry of the lignocellulose fibers to be delignified (containing for example 2 percent based on the total weight thereof of trisodium phosphate) at an appropriate consistency (for example percent based on the dry weight of the fibers) is pumped into a pressure vessel at appropriate temperature. The fibers can be separated from the aqueous trisodium phosphate in any convenient way and formed into a web for shipment with or without washing. In the latter alternative the cellulose when shipped contains substantially no lignin or trisodium phosphate.

Regeneration of the aqueous medium on recovery as described above is effected by adding to the liquor sufficient sodium hydroxide to compensate for the alkalinity taken up by the lignin as an incident in its solubilization. Since solubilization of the lignin involves conversion of trisodium phosphate to disodium phosphate, the amount of sodium hydroxide which is added is that which is needed to convert the disodium phosphate to trisodium phosphate. Since the pH of the solution declines with formation of disodium phosphate, a suitable amount can be determined by use of a pH meter, which can be arranged to provide the necessary amount of sodium hydroxide automatically.

If desired, in cyclic operation a part of the aqueous medium may be bled off from time to time to maintain the organic con- EXAMPLE 2 The following illustrates the process of the present invention as applied to three different commercial pulps and illustrates the effect of variations in the process.

In each instance a small amount of trisodium phosphate (TSP) as a concentrated solution is added to a sample of one of the pulps and the sample is diluted with water and shaken well. Amounts are shown in the table below. The resulting fibrous slurry is placed in a laboratory rocking autoclave which is sealed, evacuated, supplied with pure oxygen and heated as shown in the table. From prior experience it is known that the respective actions of the oxygen and the trisodium phosphate are substantially complete in 30 minutes, and the autoclave is cooled and opened after a longer period (as shown in the table) to insure that liberation of lignin substituents from the cellulose has substantially ceased. The pulp is then washed with water and dried.

The brightness of the pulp is determined before and after the treatment by standard laboratory photometer. The increase in brightness is a measure of the percentages of color bodies (including lignin) which are removed by the treatment.

The percentage of the lignin which is removed by the treatment is found by determining the lignin content of the pulp before and after treatment by TAPPI method T236m-60, wherein the lignin in a weighed sample of pulp is liberated with potassium permanganate solution and the solution is back titrated to determine the amount of permanganate consumed, from which the amount of liberated lignin can be calculated.

Results are as follows:

Bleaching treatment Results, percent Pulp used Fiber Percent Oz Bright- 112 TSP press, Temp, ness Lignln No. Fiber ratio 1 added 2 lb./sq. in. Hours incr. removed 4 1 By weight.

1 Weight of TSP on dry weight of fibers.

over untreated control, by photometer. Measures total color bodies removed.

text above).

3 Percent increase in reflectance By permanganate method (see tent of the aqueous medium substantially constant at at a desirable level, the trisodium phosphate in the aqueous medium thus removed being replaced by an equal amount of freshly prepared solution. A more economical method of making the replacement is to evaporate the water from the aqueous medium bled off, and to ignite the dry solid residue. The ash is principally trisodium phosphate, which can be returned to the system with the appropriate amount of makeup water. Disodium phosphate in the aqueous medium bled off can be converted to trisodium phosphate before or after evaporation and ignition. By means of the above-described procedure it becomes unnecessary to discharge any liquid effluent, and stream pollution is minimized.

The examples which follow illustrate preferred embodiment of the invention and are not to be construed as limitations thereon.

EXAMPLE 1 A dry sheet of kraft digester pulp is immersed in a 5 percent by weight solution of trisodium phosphate, and removed, and hung vertically for a minute to allow surplus solution to drip off. The sheet [containing four times its weight (dry fiber basis) of the solution, providing 20 percent of trisodium phosphate based on the dry weight of the fibers] is placed in a laboratory rocking autoclave and subjected to air under a pressure of 400 lb./in. at 95 C. for 3 hours (equivalent to an oxygen pressure of 80 lb./in. The sheet is then removed and washed with water at 20 C. to remove the phosphates and solubilized lignin and other color bodies. The sheet is substantially brightened and delignified.

EXAMPLE 3 The following illustrates the effect of variations in the pressure of the oxygen on the amount of lignin solubilized, showing the effectiveness of the process of the invention with solutions which contain dissolved oxygen at low pressure.

The procedure of run A-l of Example 2 is repeated, except that the water used has been previously boiled to drive off all gases, and the oxygen is supplied by bubbling oxygen through the aqueous medium and the temperature of the solution is increased to C. As a result, the oxygen in the solution is under a pressure of about 2 lb./sq.in.

Results are as follows.

Bleaching Treatment Bright- Run 0 ness, Lignin No. Press Hrs. Increase Removed l 2 42% 39% 2 l 2 3 57% 46% 3 4 67% 52% dissolved oxygen and trisodium phosphate by a process which 2. A process according to claim 1 wherein said aqueous medium has a temperature between about 75 and C.

A process according to cl m 3, wherein the oxygen is pure oxygen under an absolute pressure ofSO-l 50 ill/i11 4. A process according to claim l. wherein the water is a pan of the aqueous medium which is separated from said fibers is evaporated, leaving a residue of a sodiur phosphate and a sodium salt of oxidized lignin, said salt is burned, and the residue from said burning is returned to the process, the amount of aqueous medium thus separated being sufficient to maintain the concentration of said salt in said process substantially constant. 

2. A process according to claim 1 wherein said aqueous medium has a temperature between about 75* and 150* C.
 3. A process according to claim 1, wherein the oxygen is pure oxygen under an absolute pressure of 50-150 lb./in.2.
 4. A process according to claim 1 wherein the water is a part of the aqueous medium which is separated from said fibers is evaporated, leaving a residue of a sodium phosphate and a sodium salt of oxidized lignin, said salt is burned, and the residue from said burning is returned to the process, the amount of aqueous medium thus separated being sufficient to maintain the concentration of said salt in said process substantially constant. 